1. Field of the Invention
This invention pertains to digital data communication systems.
2. Description of the Prior Art
Communication networks typically consist of a plurality of communications processing nodes connected via communication links, be they leased telephone lines, local area networks (LANs) or wideband span lines operating at 1.544 Mbps or higher. In addition, it is very common to initially deploy a limited number of nodes serving initially some limited number of mainframes and users, terminals, voice trunks and the like.
As a network application matures, the number of nodes invariably increases, and each node grows to support an ever-increasing number of customer-premises data processing and communications equipment. It invariably forces the users to obsolete the communications processing nodes, because nodes with sufficient reserve processing power are not purchased initially due to cost.
Lines to be serviced by a node are generally of two kinds: "CPE (customer premises equipment) side" and "network side". The CPE-side lines connect to and service such equipments as local terminals, PC's, LAN's, PBX's, and so forth, while the network-side lines connect the particular node to other nodes in the network. In general, lines coming in on the CPE side may carry data at relatively low data rates, while network-side lines are usually high-speed lines (e.g., T1 lines operating at 1.544 Mbps). It is customary to allocate data rates to network-side line bandwidths in 64 Kbps "slots", which means that a low data rate transaction makes inefficient use of such a slot. For a transaction requiring more than one slot, the prior art generally imposes the constraint that contiguous slots must be allocated; this sometimes leads to call blocking even though sufficient slots are available, because the available slots are not contiguous.
It is also customary for a node to allocate half of a line's bandwidth to transactions that it (the node) may allocate to that line, while reserving the other half for transactions that the node on the other end of that line may allocate to that line; under this arrangement, when a preponderance of transactions is being initiated from one "side" the allocation may be exceeded and calls will have to be blocked, even though unused capability remains allocated to the "other side".